fix(runtime): drop tip on RealPaysFee proxy fee-payer path

[loom-agent]

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Release Notes

• Fixed a delegate-drain vulnerability in the RealPaysFee proxy path where a delegate could attach an arbitrarily large tip and bill it to the real account.

Problem

In the RealPaysFee proxy fee-payer path, the tip chosen by the signer (the delegate) is passed through via self.inner.tip() and billed to the real account. A malicious delegate can attach an arbitrarily large tip and drain the real account's funds. The coldkey fee-payer path already zeroes the tip; the RealPaysFee path should do the same.

Root Cause

In runtime/src/transaction_payment_wrapper.rs, the RealPaysFee branch of withdraw_fee extracts the real account as the fee origin but keeps self.inner.tip() as the tip. The delegate, not the real account, chooses the tip.

The Fix

Replace self.inner.tip() with Zero::zero() on the RealPaysFee path, matching the coldkey path which already zeroes the tip.

Testing

Added audit_probe_real_pays_delegate_selected_tip which submits a proxy call with a 1_000_000 tip. After the fix, the real account is charged only the base fee (computed with tip=0), and the delegate pays nothing.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

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